This invention relates to an active matrix-addressed display device. More particularly, the invention is concerned with an active matrix-addressed liquid crystal display device comprising a row and column array of display elements having first and second spaced electrodes with liquid crystal therebetween, a plurality of row and column conductors to which switching signals and data signals are applied respectively with each row conductor being associated with two adjacent rows of display elements, each display element being associated with two transistors connected to two different row conductors for controlling the application of data signals to tis first electrode and having a respective storage capacitor connected to the outputs of the two transistors.
Active matrix-addressed display devices using switching transistors are well known. In their simplest form, a single transistor acting as a switch is sued for each display element with its drain electrode connected to an electrode of the element. The source electrodes of all transistors in the same column are connected to a respective one of a set of column conductors to which data signals are applied and the gate electrodes of all transistors in the same row are connected to a respective one of a set of row conductors to which switching (gating) voltages are applied to control switching of the transistors. The row conductors are conductors are scanned one at a time in sequential fashion with the switching voltage so as to turn on all transistors in each row in turn, with data signals being applied to the column conductors appropriately in synchronism for each row in turn, so as to build up a display. When the transistors are on, the data signals are supplied to the associated display element electrodes, thus charging up the display elements. When the transistors are turned off, upon termination of the switching voltage, and the application of a suitable "off" voltage to the row conductor, the charge is stored in the display elements concerned until the next time they are addressed with a switching voltage, usually, in the case of a video display, in the next field period.
Such an active matrix addressed liquid crystal display device is capable of displaying TV pictures. The transistors may be bulk transistors, e.g., MOSFETs, formed on a semiconductor substrate of the device. For large area display devices, TFTs deposited on a glass substrate are normally used. With the increasingly larger display areas now being proposed, there comes a corresponding increase in the number of display elements, and hence switching transistors, required.
A major problem in making large area display devices of this kind is yield. A discontinuity in a row conductor or just a few defective transistors can render the device unacceptable. Depending on the nature of the defect, even one defective transistor can lead to complete rows or columns of display elements being unusable.
The kind of display device mentioned in the opening paragraph, having two transistors for each display element, which are connected to respective, different, row conductors, is advantageous in this respect as a degree of redundancy is afforded. Considering one display element, and assuming both associated transistors and their respective row conductors are not faulty, one transistor operates to load the display element with a data signal voltage during a row address period and thereafter, during a subsequent row address period, the other transistor operates to over-write the data voltage with fresh data signal voltages until the one transistor is again operated. In the event of one of the two transistors proving to be defective or one of the two row conductors concerned having a discontinuity which leads to an open circuit condition, the display element is nonetheless driven to provide a display effect by virtue of the operation of the other transistor. Although two successive display elements in the column concerned may therefore produce the same display effect this is hardly perceptible. A display device of this kind is described in GB-A-2115199.
Liquid crystal materials typically exhibit less than ideal resistivity and dielectric properties. Accordingly, to compensate for charge leakage in the display elements between addressing, storage capacitors are often desirable for acceptable display performance, particularly for a display device which is to be used in a projection LC-TV system where deterioration in the transmission characteristics of a display element over a field period would be more noticeable. In the display device described in GB-A-2115199, a storage capacitor is provided for each display element, with one side of the storage capacitor being connected to the drains of the two transistors associated with the display element. The other side of all the storage capacitors associated with the rows of display elements are interconnected and maintained at a fixed potential. Although this arrangement in theory provides operational advantages, the provision of such storage capacitors necessarily complicates fabrication of the display device. Indeed, GB-A-2115199 is silent as to how these capacitors could be formed.